1. Field of the Invention
The present invention relates to the preparation, via particular anionic polymerization, of block copolymers comprising polydiene blocks and polymethyl methacrylate blocks. The subject block copolymers are thermoplastic elastomers and have improved heat resistance and impact strength.
2. Description of the Prior Art
It is known to this art to prepare thermoplastic elastomeric copolymers, especially SBS (polystyrene/butadiene/styrene) and SEBS (polystyrene/hydrogenated butadiene/styrene). However, their limiting temperature of application is less than 70.degree. C. and 80.degree.-100.degree. C., respectively. Beyond these limits the copolymer softens and loses its shape.
Attempts have been made to prepare thermoplastic elastomeric copolymers comprising at least one block having a higher glass transition temperature. Thus, Belgian Patent 759,737 describes thermoplastic elastomeric copolymers of formula A-B-C in which A and/or C is an inelastic polymer sequence prepared from acrylic monomers and which has a glass transition temperature higher than 100.degree. C. These blocks have an average molecular weight ranging from 200 to 20,000, preferably less than 18,000 and, in particular, less than 15,000, especially polymethyl methacrylate (Mn =1,600 in the Table). The block B may be prepared from a conjugated diene such as butadiene, preferably butadiene having a high content of cis-1,4-configuration. To prepare these copolymers, via anionic polymerization, an initiator of the alkyllithium type is employed in an apolar solvent medium, such as cyclohexane.
It will thus be appreciated that the average molecular weights of the acrylic blocks described in this '737 patent are low, which presents the disadvantage of decreasing mechanical strength. This may be explained by the fact that the efficiency of initiation of polymerization of methyl methacrylate (MMA) in a purely apolar medium (cyclohexane) is very low, even at very low temperature (-78.degree. C.) and that secondary reactions ensue due to the attack by the anions of the ester groups of the MMA monomer present in solution, or on the polymethyl methacrylate chains (PMMA) already formed. Indeed, when the MMA monomer is present with the butadiene anion, for example, polymerization occurs, but the anion preferentially attacks the esters rather than the double bond. A short PMMA polymer chain is therefore obtained. Moreover, this '737 patent is silent in respect of glass transition temperatures (Tg). Under the conditions described for the preparation of such polymers, the PMMA block should be isotactic and should have a low Tg, for example 60.degree.-70.degree. C.
FR-A-2,679,237, assigned to the assignee hereof, describes an initiator system for the anionic polymerization of (meth)acrylic monomers to prepare, in particular, block copolymers comprising hard/soft/hard blocks and also PMMA structural units containing a high percentage of syndiotactic triads, imparting a glass transition temperature on the order of 130.degree. C. to the polymer. The preparation of copolymers comprising polymethyl methacrylate and polydiene blocks is not described.
Polymers comprising syndiotactic polymethyl methacrylate and polybutadiene blocks are also known to this art. However, their preparation requires a change of solvent (for example toluene and then THF) and the reaction is carried out at low temperature (-78.degree. C.). These conditions are not suitable on an industrial scale. Moreover, the addition of large amounts of THF, for example after the formation of the polybutadiene "living" structural unit may result in its deactivation by reaction with the residual impurities present in the solvent. This results in the formation of an impure product (mixture of copolymer and homopolymer).